In general, the plastic packaging of ICs is as follows: Typically, ICs in die form are attached to mounting areas called islands on strips called lead frames. The lead frames are made of a thin, flat, electrically conductive material and typically have several individual islands, each for supporting an individual IC. In most cases, densely packaged ICs are manufactured to maximize connectivity by utilizing all four sides of the chip.
Around the perimeter of each island a lead frame has a pattern of individual conductive leads extending toward, but not contacting the island. The islands and individual leads are formed by selective removal of material in the lead frame, such as by stamping. The number of the leads at a frame depends directly on the configuration of the particular IC die to be mounted. A typical IC may have over one hundred external connections and each frame will have a corresponding number of individual leads. The width of each lead and the separation between adjacent leads is dependant on the package size of the finished IC. The thickness of each lead is the thickness of the lead frame and is predicated on the current carrying capacity required.
A plastic package with external leads for connecting to, for example, a printed circuit board, is typically formed by an encapsulation process. Mating molds are placed on each side of the lead frame and liquid-phase polymer is injected to encapsulate the IC die. The lead frame is designed to dam the flow of liquid-phase polymer as it moves to the outer edges of each individual mold, stopping at where each mold contacts surfaces of the lead frame. To stop the flow of liquid-phase polymer between leads the lead frame has a pattern of dam bars between individual leads, so a contiguous band of material is formed around the periphery of the island. This contiguous band prevents the polymer from flooding the leadframe.
After the polymer solidifies and the molds are removed, a following operation in the manufacturing process removes the excess plastic in the region around the mold outline and the dam bars. This is termed de-junking in the art. A de-damming process then removes the dam bar between each lead, providing electronic integrity for each lead. De-damming is a process of removing all or part of each dam bar by use of a punch with a pattern of teeth conforming to the pattern of the dam bars in the lead frame. Typically, the de-damming and de-junking can be done in a single step. In following processing each lead exposed from the edge of the plastic package is further treated such as by plating, and the individual packages are trimmed from the lead frame strip. Finally, the leads are formed, such as for Surface Mount Technology (SMT) applications.
In state-of-the-art manufacturing, automated machines perform the de-junking and de-damming operations. Automatic machines are marketed by Iwtani International Corporation of Tokyo, Japan and Fujitsu of Japan, among others. In the de-damming operation, typically a hydraulically driven, hardened metal punch is used to trim the dam bar from between the conductive leads. The punch is critically machined to provide a clean cut for each dam bar to insure physical dimensions and electrical integrity. In one case, the de-damming punch is designed to cut the dam bar between every second pair of leads of an IC package. This is done to minimize the manufacturing cost of the punches used for de-damming. In most instances, an automated machine will have two opposing dam bar punches working in unison on opposite sides the package. In this case, the complete de-damming operation may take up to four stages to trim all dam bars from each package.
The dam bar punches are produced uniquely for specific IC packages since many ICs have different lead counts, lead pitches and package sizes. The punches must be manufactured to maintain functional integrity over many cycles. As pitch sizes get smaller so do the de-damming punch's individual teeth that clear the dam bar between individual leads. When a tooth breaks, the entire punch must be replaced. The broken punch is typically discarded because it is more economical to buy a replacement than to try to repair/re-machine a new set of teeth on the same punch.
As described above, de-damming punches are typically made to punch every second dam bar. This is done to control manufacturing costs for the punches, as it allows wider spacing between individual teeth, and punches with wider spacing are less expensive to produce. There is a disadvantage, however. As IC lead aspect ratios decrease because of the higher pin counts and smaller packages, shearing forces increase and tend to twist the alternately punched leads. Then, when the lead is formed for a SMT application, the lead pad positioning may be offset, decreasing production yields.
What is needed is a de-damming punch compatible with existing state-of-the-art automated machinery that can be cost effectively reworked rather than discarded. Such a punch should preferably punch every dam bar on a side in one action, assuring coplanarity in pad positioning. This would save money by eliminating the need to provide a new punch every time a punch fails in use.